Homozygosity for histocompatibility genes was attained at the 22- 25th and 18th generations of the Chinese and Armenian hamster, respectively. Presently, the most advanced strain of the Chinese hamster is at the 39th generation of brother-sister matings; while its counterpart for the Armenian hamster is at the 25th generation. The most critical period of inbreeding both species is the 22-25th generations. Over 95 percent of the sublines are eliminated at the time. Losses, starting at around the 18th generation, results in numerous sublines failing to reproduce. In both species homozygosity for histocompatibility genes was accompanied by an abrupt shift in the cell types implicated in spontaneous neoplasia. Prior to the 18th generation, tumors stemmed from mesenchymal cells (fibrosarcomas), whereas coincidental to achieving 95 plus percent level of genic homozygosity, tumors of both species now stem from lymphoid and reticuloendothelial cells. Retention of the genotype for maturity-onset diabetes mellitus, identification of an animal subline bearing a spontaneous reciprocal translocation, and assessment of both species in carcinogenic trials employing nitroso compounds have also been initiated this year.